Various types of operations are routinely performed today on human joints, for example the knee joint, which involve partial or total replacement of the bone end surfaces involved in the joint. For example, knee joint replacement might involve resectioning the distal femur in order to prepare it to receive a prosthetic device.
As used herein, when referring to bones or other parts of the body, the term “proximal” means closer to the heart and the term “distal” means more distant from the heart. The term “inferior” means toward the feet and the term “superior” means towards the head. The term “anterior” means towards the front part of the body or the face and the term “posterior” means towards the back of the body. The term “medial” means toward the midline of the body and the term “lateral” means away from the midline of the body.
A basic requirement for such resectioning is that the mechanical operation of the knee joint must be preserved. That is, the operation must not change the plane in which the leg pivots about the knee joint. This requirement is commonly expressed as preserving the “mechanical axis” of the leg. This mechanical axis is a straight line which passes through the center of the femur head, the center of the knee joint, and the center of the ankle.
The anatomical axis of the femur is a line which passes through the center of the shaft portion of the femur. It is a well-known practice to use the anatomical axis in order to locate the mechanical axis prior to resectioning the distal femur. This is accomplished by inserting a rod from the distal end of the femur into the intramedullary canal of the femur. Such a rod provides a close approximation of the anatomical axis.
Various types of guides have been known which mount on such an intramedullary rod and then permit a surgeon to locate the mechanical axis for distal femur resection. See for example, U.S. Pat. Nos. 4,759,350; 5,688,281; and 6,193,723. In using these known devices, the angle between the patient's mechanical axis and his femoral anatomical axis, hereafter referred to as the “femoral offset angle”, is determined ahead of time, for example from an x-ray of the leg. Once the cutting guide is mounted on the intramedullary rod, the orientation of the cutting guide is adjusted in order to preserve the femoral offset angle. After the appropriate adjustment is made for the femoral offset angle, it is necessary to secure the cutting guide in position, as by pinning to the bone, before the resection can be performed. At this point resection can proceed in a cutting plane which is perpendicular to the mechanical axis.
Rather than forcing the surgeon to use the cutting guide in a fixed position, it would be desirable to permit him to move the cutting guide once its position has been established relative to the intramedullary rod, while still maintaining it in the cutting plane, so that the surgeon can position it in order to achieve maximum exposure of the bone. It would be even more desirable to permit the cutting guide to be moved relative to the bone while the cut is being made while maintaining it in the cutting plane. This would permit the surgeon to continually maintain an optimum view of the guide, the cutting tool, and the bone.